Line a 9×9 pan with nonstick foil, or line with normal foil and spray with nonstick spray. Set aside.
Bring heavy cream, vanilla, and salt to a boil in a small pot. Turn off heat and leave pot on the stove.
In a large pot, add sugar and corn syrup. Cook on medium heat, stirring only to moisten the sugar. Wash down the sides of the pot with a pastry brush and water in the beginning if there are sugar crystals on the sides of the pot.
Do not stir constantly. Instead, only stir when the sugar starts to caramelize. You will see the sugar turn a light amber color.
Let the sugar caramelize until it is a deep amber color, similar to the color of almond skins.
Turn the heat off and pour in the hot heavy cream while slowly whisking. It will bubble violently, so be sure to add the cream slowly so it doesn’t bubble onto your hands.
Return to medium heat and attach candy thermometer to the pot. Stir frequently and bring temperature to 248 degrees F.
As soon as the caramel reaches firm ball stage, remove from heat and stir in the butter until fully melted and combined.
Pour caramel into lined pan and smooth caramel out evenly. Sprinkle with fleur de sel at this point, or you can wait until the caramel has been sitting for an hour and is a bit firmer.
Let caramel cool for 2-3 hours. Remove foil sling from pan and gently peel foil away from caramel. Cut caramel into 1-2 inch pieces. Store between parchment paper or wrap caramels individually for optimal freshness. Enjoy caramels within 1-2 weeks.
Keywords: soft caramel recipe, salted caramel, caramel with fleur de sel.